6th
International Science, Social Sciences, Engineering and Energy Conference
17-19 December, 2014, Prajaktra Design Hotel, Udon Thani, Thailand
I-SEEC 2014
http//iseec2014.udru.ac.th
Modification of electrode surface by polydimethylsiloxane –
gold nanoparticle film for electrochemical biosensor
Rachatawan Kamolpacha,e1, Chokchai Puttharugsa b,e2
ab
Department of Physics, Faculty of Science, Srinakharinwirot University,
114 Sukhumvit 23, Bangkok 10110, Thailand
e1
rachatawan_som-o@hotmail.com’s email , e2chokchai@swu.ac.th’s email
Abstract
The gold screen printed electrode (AuSPE) were modified surface by polymer nanocomposite of
polydimethylsiloxane - gold nanoparticle (PDMS-AuNP) for using in the electrochemical biosensor. The
in-situ synthesis of PDMS-AuNP composite film was simply prepared by dropping HAuCl4 solution on the
PDMS film surface. The PDMS-AuNP synthesis were characterized by using UV/vis spectrophotometer,
field emission scanning electron microscope (FE-SEM), cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS) technique. The AuNP size on the PDMS film was 1-50 nm, approximately. The
PDMS-AuNP modified on AuSPE surface can be applied and used as a supporting layer for further
biological immobilization in electrochemical biosensor.
Keywords: polydimethylsiloxane - gold nanoparticle (PDMS-AuNP), gold screen printed electrode (AuSPE), electrochemical
impedance spectroscopy (EIS).
1. Introduction
An electrochemical biosensors have been widely used for biological detections such as bacteria,
virus, enzyme, and DNA detections [1]. With the properties of high sensitivity, fast responds and low cost,
the electrochemical biosensors continue to make significant impact, especially in biological and medical
applications. With the recent advances in nanotechnology, nanomaterials have received great interests in
the field of biosensors due to their exquisite sensitivity in chemical and biological sensing [2]. The
nanomaterials were used in biosensors for example, the use of nanoparticles, nanorod, nanowire, nanotube,
nanoporous, nanofibers, nanobelts and thin films as modification of electrode surface for enhancing the
sensitivity of biosensor [3]. Gold nanoparticles (AuNP) have received greatest interests in modification of
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electrode surface because they have several kinds of intriguing properties, such as a high surface-to-volume
ratio with excellent biocompatibility, high sensitivity, and high stability [4-6] immobilization of a large
amount of biomolecules retaining their bioactivity. Moreover, the modified electrode surface with AuNP
has an advantages with providing fast and direct electron transfer between a wide range of electroactive
species and electrode materials [7].
Poly(dimethylsiloxane) (PDMS) is one of the most widely used polymer materials for fabricating
microfluidic chips due to its transparency, outstanding elasticity, good thermal and oxidative stability, ease
to be fabricated and sealed with various materials [8,9]. In 2008, Qing Zhang and colleague presented a
simple approach for in-situ synthesis of polydimethylsiloxane-gold nanoparticle (PDMS-AuNP) composite
film [10]. They simply prepared the PDMS-AuNP by dropping gold salt on the PDMS. The gold ion can
be reduced to AuNP using the remaining of curing agent. The size of AuNP can be simple controlled by
adjusting the ratio of curing agent and the PDMS monomer. Moreover, the experiments have shown that
the resulting composite film may have a lot of potential merits in protein immobilization, immunoassays
and other biochemical analysis using PDMS-AuNP. In 2010, Pooja Devi and colleague, they are synthesis
and surface modification of poly(dimethylsiloxane) – gold nano composite films used to study the binding
of Human Serum Albumin (HSA) to polyclonal anti-HSA. Biosensing experiments carried out with the AuPDMS composite showed a good sensitivity allowing the detection of 2.5 g of antigen [11]. The method is
suitable for microfluidic sensing applications for a variety of biomolecules. The same year, Wen-Ya Wu
and colleague developed the colorimetric detection based PDMS-AuNP using silver enhancement for
detection of cardiac troponin I [12]. The detection limit was 0.01 ng/ml. In the assay, antibody was
immobilized on the PDMS-AuNP surface and bovine serum albumin was used as a blocking. The amount
of antigen detection was relative to the darkness of silver deposition. Recently (in 2013), Hamid SadAbadi
and colleague demonstrated the PDMS-AuNP microfluidic chip and used LSPR-based biosensor for
detection of polypeptides [13]. The proposed biosensor can reach a detection limit of as low as 3.7 ng/ml.
This results demonstrate the successful integration of PDMS-AuNP composite film based biosensors which
provide a potential alternative for biological detection in applications.
The main objective of this work is to synthesize the PDMS-AuNP nanocomposite film on the
electrode surface. The major emphasize will be on the simply synthesis by dropping HAuCl4 on the coated
PDMS surface and study effect of PDMS concentration and ratio between curing agent and monomer in
PDMS. In addition, the PDMS-AuNP film were characterized by UV/vis spectroscopy and contact angle
analysis. We expected that PDMS-AuNP coated AuSPE surface may be applied to electrochemical
biosensor techniques for further development of biosensors with high sensitivity and stability.
2. Methodology
2.1. Materials
Gold screen printed electrode (AuSPE) was purchased from DropSense (Spain). Detail of electrodes
include a gold working electrode (disk-shaped, 12.6 mm2), a gold counter electrode, and a silver pseudoreference electrode. The electrodes are screen-printed on ceramic substrate and curing at low temperature.
PDMS, silicone elastomer base and silicone elastomer curing agent, was purchased from Dow corning,
Thailand. Potassium dihydrogen phosphate (KH2PO4), sodium acetate (CH3COONa·3H2O), di-sodium
3
acetate (CH3COONa.3H2O), sodium phosphate monobasic (NaH2PO4·H2O) and potassium chloride (KCl)
were purchased from Caro erbe, Thailand
2.2. PDMS preparation and PDMS-AuNP synthesis
PDMS was prepared by mixing the silicone elastomer base and elastomer curing agent at a ratio of
10:1.0 (w/w). The mixed PDMS was degassed and then diluted at different concentrations (0.1, 0.5, 1.0,
5.0, 10 and 20% w/v) in propane. The glass slides were cut in a piece of 1.0×1.0 cm 2 and were cleaned by
rinsing with water, ethanol and dried with N2. For fabrication of PDMS film, the 100 µL of diluted PDMS
solution was dropped on the glass substrate and was spun 1,000 rpm for 60 s by using a home-made spincoater. For PDMS-AuNP synthesis, 100 µL of 20 mg/ml HAuCL4 was dropped on the PDMS films with
24 h of incubation time. The PDMS-AuNP surfaces were characterized by using UV-vis spectrophotometer
(Lambda 25 UV/vis spectrometer, Perkin Eimer istruments). The optimized PDMS concentration was
further used in other optimized factors. The effect of incubation time, the optimum condition of PDMS
concentration was used. 100 µL of 20 mg/ml HAuCL4 was dropped with different incubation times (2, 5,
8, 12, and 24 h).
2.3. Effect of  ration
The sized of gold nanoparticle was controlled by the ratio of silicone elastomer base and curing agent
().The ration  were prepared in different ratio 10:0.6, 10:1, 10:1.5, 10:1.8, and 10:2 (w/w) denoted as 
= 0.6, 1, 1.5, 1.8, and 2. The PDMS-AuNP films were synthesized as mention above with different  of
PDMS.
2.4. Effect of HAuCl4 concentration
HAuCl4 was diluted in deionized water (DI water) at different concentrations (1, 5, 10, 20 and 40
mg/mL). The optimized PDMS surface were selected for investigation in effect of HAuCl 4 concentrations.
The different concentration of HAuCl4 (200 µL) were dropped and incubated on the optimized PDMS
coated on glass substrate for 24 h. The PDMS-AuNP was then analyzed and characterized by UV-vis
spectroscopy.
2.5. Effect of incubation time
The 20 mg/ml of HAuCl4 was dropped and incubated on the optimized PDMS surface for 2, 5, 8,
12, and 24 h of incubation time. The synthesis of AuNP on PDMS surfaces were then characterized by
mean of UV-vis spectroscopy. The information of optimized time, PDMS concentration, and HAuCl4
concentration was around 24 hours for 20% of PDMS incubated in 20 mg/ml of HAuCl4 for further
preparation of AuNP-PDMS on the AuSPE surface.
2.6. CV and EIS measurement
The AuSPE was cleaned by rinsing with DI water and dry with N2 gas. 10 µL of PDMS solution
was dropped on the AuSPE surface and was spun at 1,000 rpm for 60 s. The PDMS film on AuSPE was
4
then cured at 60 degree for 30 min. The HAuCl4 was then dropped on the PDMS surface for synthesis of
gold nanoparticle. The Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were
used to characterize the film properties. CV and EIS were measured by using a µ-Autolab with FRA2 (ECO
Chemie). All the experiment were carried out at room temperature (25  1 C). CV and EIS measurement
were carried out in the presence of 5 mM [Fe(CN)6]3-/4- redox probe solution. CV was performed in the 0.25 to 0.5 V range at a scan rate 100 mV/s for monitoring PDMS and PDMS-AuNP film. Impedance
measurement was recorded in range from 0.1 – 10,000 Hz of frequency. The alternating voltage was
constant at 10 mV in amplitude. The impedance results were presented in a form of Nyquist plot. It plots
between real part and imaginary part of resulted impedance. The Nyquist plots were fitted with proper
equivalent circuit model using the facility of NOVA software v. 10.0 (ECO Chemie).
3. Results and discussion
The UV-Visible absorption spectroscopy was used to monitor the surface plasmon absorption of
gold nanoparticles. Figure 1 shows the UV-Vis spectra of the different PDMS films incubated with the
HAuCl4 solution for 24 h. The synthesized gold nanoparticle appeared when the concentration of PDMS
was greater than 5% w/v. The center of absorption band was 541–547 nm resulting from the surface
plasmon band of gold nanoparticle [9]. The absorption intensity increased with concentration of PDMS,
indicating the formation and increasing population of gold nanoparticles. Generally, the crosslink of PDMS
is based on the reaction between silicon hydride (Si – H) group in the curing agent and vinyl group (Si –
CH=CH2) in the monomer as shown in the equation 1. In cured PDMS, There are remaining of silicon
hydride groups which are the precursors for synthesis of gold nanoparticle.The possibility of reaction
between gold ion and silicon hydride groups has been suggested by Qing Zhang[10] as shown in the
equation 2. This suggests that the population of gold nanoparticle depends on the amount of silicon hydride
groups. Moreover, the 20% of PDMS concentration showed the highest peak absorbance due to the high
amount of residue silicon hydride group on the surface. Figure 2 shows the schematic of PDMS-AuNP
synthesis.
(1)
(2)
Figure 2. Schematic of PDMS-AuNP synthesis.
Figure 1. UV-Visible spectra of different concentration
incubated in 20 mg/mL of HAuCl4 solution 24h.
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Figure 3 shows the difference of incubation time for synthesis of PDMS-AuNP. The adsorption intensity
increased with incubation time and reached saturation at 12 h. This indicate that the formation and
population of gold nanoparticle was appeared on the PDMS film. Due to the thin layer of PDMS on the
surface, the amount of synthesized gold nanoparticle is limited by the amount of remaining of curing agent.
The dependency of absorbance resulting from the different HAuCl4 concentrations is shown in figure4. The
absorbance peak (𝜆max = 539) increased with proportional to the HAuCl4 concentration. At 20 mg/ml of
concentration, the absorbance peak was maximum for the synthesis of gold nanoparticle. This suggests that
this concentration is suitable or have high enough concentration for further synthesis of PDMS-AuNP on
the surface.
Figure 3. UV-Visible spectra of different time incubated
Figure 4. UV-Visible spectra of different concentration
in 20 mg/mL of HAuCl4 solution for 20% PDMS surface. . HAuCl4 in 20% PDMS surface for 24 h of incubation time.
Figure 5 shows the difference of  in which the PDMS surface was immersed in 20 mg/mL HAuCl4 aqueous
solution for 24 h of incubation time. The differences of  can also be observed in the UV-Vis spectra. The
maximum absorption peak shifts from 538 nm to 540 nm, with the increase of  from 0.06 to 0.2. The 
was increase yielding the more residual Si–H groups existed on the PDMS surface. These Si–H groups can
be quickly react with AuCl-4 anions on the surface and becomes nanoparticles. The difference of  affect
the particle size see from the shifts of maximum absorption peak. Figure 6 shows the FE-SEM image of
PDMS-AuNP in different  ration. This ration has a great effect on the shape and size of the gold
nanoparticles as seen in figure 6. When  was 0.1 (fig.6a), the size of AuNP average of about 1-200 nm in
range, approximately. The shape of most nanoparticles seemed to be round. When  was 0.15 (fig.6b), the
size of AuNP the average of about 10-400 nm in range, approximately. The nanoparticle shape was changed
to polygonal, including triangles, ellipses, pentagons and hexagons.
Figure 5. UV-Visible spectra of different  incubated
in 20 mg/mL of HAuCl4 solution for 24 h.
Figure 6. FE-SEM images of the PDMS–gold nanoparticles
free-standing films with (a) =0.1, (b) =0.15
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The PDMS and PDMS-AuNP films were monitored by using EIS and CV technique. Figure 7 shows cyclic
voltammogram using the 5 mM of [Fe(CN)63−]/[Fe(CN)64−] solution as a redox probe. The working
electrode modification with a 20% PDMS resulted in a decrease in the peak current as shown in figure 7
(red line). The current peak also increased in the separation of the peak potentials (red line) when compared
to the CV of an unmodified electrode (AuSPE, black line). The decrease of peak current depend on the
concentration of PDSM on the electrode surface (data do not shown). At 20% PDMS, the current peak was
disappear. This results indicate that the PDMS film was impede the electron transfer between working and
counter electrode. After drop of 20 mg/mL HAuCl4 on a 20% PDMS surface for 24 h, the gold nanoparticle
was then formed on the PDMS surface. The reduction of current peak was thus increase as shown in figure
7 (blue line) and was about 24 µA due to the formation of AuNP.
Figure 8 shows the obtained Nyquist plots for concentration of PDMS and the inset shows the Randle circuit
model. The Nyquist plot was fitted by using Randle circuit model to obtain the electron transfer resistance
(Ret). The Ret depends on the dielectric at the interface of electrode. The Ret is very sensitive to the
electrode surface modified with PDMS layer. In Figure 8, the bare AuSPE showed the fast electron transfer.
After drop difference concentrations of PDMS on working electrode surface, the value of Ret was increase
due to the layer of PDMS impeding the electron transfer (data do not shown). The Ret of 20% PDMS on
the AuSPE electrode was 14 kΩ. After drop of 20 mg/mL HAuCl4 on PDMS, the Ret was decrease to 680Ω
by the reason of AuNP formation and resulted in a good electron transfer between working and counter
electrode. The results of Nyquist plot are also consistent with the cyclic voltammogram observed in the CV
measurement. These results indicate that the AuNP can promote the electron transfer between that
electrodes.
Figure 7 Cyclic voltammogram.
Figure 8 Nyquist plot
4. Conclusions
The nanocomposite film of PDMS-AuNP was successfully prepared by a simply drop of HAuCl4
solution on PDMS film surface. In application of electrochemical sensor, the suitable condition for PDMSAuNP synthesis was 20 mg/ml of HAuCl4 incubated on the 20% of PDMS (=0.1) coated on surface for
24 h. The size of synthesized AuNP on the PDMS surface was 1-50 nm in range. The PDMS-AuNP
nanocomposite films on AuSPE can be applied in biosensor techniques for further development of EIS
biosensors in order to enhance the sensitivity and stability.Acknowledgements
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The author would like to thank the financial support by Faculty of Science and Graduate School at
Srinakharinwirot University.
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